YOU might think that the feel of a surface is what tells you how rough it is.
But it turns out that things are not that simple. The sound you hear when you
scratch a surface can interfere with what your fingers are trying to tell you,
says an Oxford psychologist, who has measured the effect for the first time.
People get genuinely confused when presented with a conflict between hearing
and touch, says Donna Lloyd of the John Radcliffe Hospital. It takes them longer
to make judgements about texture, and the whole effect is generally
disconcerting.
To quantify the effect, known as the parchment skin illusion, Lloyd asked
blindfolded subjects to judge the roughness of sandpaper. At the same time, the
sound of the surface being rubbed was relayed to them through headphones. By
turning up the high frequencies, Lloyd could make the surface sound smoother.
Turning down those frequencies made it sound rougher.
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Even though the subjects were told to ignore the sounds, they were
significantly influenced by what they heard. When presented with a conflict, it
took people an average of 80 milliseconds longer to judge the roughness, a full
10 per cent increase in the time to make a decision. “It’s quite a strong
sensation,” says Lloyd, who says she was surprised by the effect. However, the
subjects still generally got the answers right.
“There’s no situation where you’re using only one sense,” she says. She adds
that this crossing of sensory inputs can change our behaviour, even when we’re
not aware of it. Lloyd and her group plan to study the effect through functional
magnetic resonance imaging.
“We don’t like things that sound noisy or plasticky,” Lloyd says. And that
could affect our buying habits in everything from clothes to furniture.